Atomic Mass Chemistry Example 2

Follow the full solution, then compare it with the other examples linked below.

Example 2

medium

Silicon has three stable isotopes:

{}^{28}\text{Si}

(92.23%, mass

27.977

{}^{29}\text{Si}

(4.67%, mass

28.976

), and

{}^{30}\text{Si}

(3.10%, mass

29.974\,\text{amu}

). Calculate the atomic mass of silicon.

Solution

1
Use the weighted average formula: $\bar{m} = \sum (m_i \times f_i)$ .
2
$\bar{m} = (27.977 \times 0.9223) + (28.976 \times 0.0467) + (29.974 \times 0.0310)$ .
3
$\bar{m} = 25.803 + 1.353 + 0.929 = 28.09\,\text{amu}$ .

Answer

\text{Atomic mass of Si} = 28.09\,\text{amu}

This three-isotope calculation demonstrates the general method for computing atomic mass. The result is heavily weighted toward the most abundant isotope (

{}^{28}\text{Si}

at 92.23%), which is why the atomic mass is close to 28.

About Atomic Mass

The weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu), where each isotope's mass is weighted by.

Learn more about Atomic Mass →

More Atomic Mass Examples

Example 1 easy

Define atomic mass and explain why the atomic mass of carbon is listed as [formula] rather than exac

Example 3 medium

Chlorine has two stable isotopes: [formula] (mass [formula], 75.77%) and [formula] (mass [formula],

Example 4 hard

An element has two isotopes. Isotope A has mass [formula] and isotope B has mass [formula]. The atom

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Isotope Molar Mass